Communicating digital control and message data signals over radio communications channels is already well known in the art. See, for example, the following (by no means exhaustive) list of prior-issued United States Patents:
U.S. Pat. No. 4,027,243 to Stackhouse et al (1977) PA1 U.S. Pat. No. 4,369,443 to Giallanza et al (1983) PA1 U.S. Pat. No. 4,434,323 to Levine et al (1984) PA1 U.S. Pat. No. 4,322,576 to Miller (1982) PA1 U.S. Pat. No. 4,267,592 to Craiglow (1981) PA1 U.S. Pat. No. 3,801,956 to Braun et al (1974) PA1 U.S. Pat. No. 4,418,425 to Fennel, Jr. et al (1983)
Commonly-assigned U.S. Pat. No. 4,027,243 to Stackhouse et al describes a digital message generator for a digitally controlled radio transmitter and receiver in a radio communications system. Provisions are made in this communications system for acquiring bit synchronization as well as word synchronization in each of a steady succession of digital command messages transmitted between radio station sites.
Commonly-assigned U.S. patent application Ser. No. 661,733 of Szczutkowski et al filed Oct. 17, 1984 describes a format of control and encoded voice digital signals which provides selective signalling capability, late entry, and word and cryptographic synchronization recovery--as well as fade and noise protection--in the context of a digital voice privacy radio communications system. Prior art FIG. 1 shows the preferred time sequence of digital signals transmitted and received by the communications system described in the Szczutkowski patent application. While additional details relating to these digital signal sequences may be learned from the specification of application of Ser. No. 661,733, the sequence shown in FIG. 1 will now be briefly described.
The sequence of digital signals shown in FIG. 1 includes a preamble followed by one or more data frames. The preamble contains data providing bit/frame synchronization, repeater addressing, cryptographic synchronization and selective signalling control. Data frames carry their own synchronization data and also contain digitized, encrypted voice or other data signals.
The signalling format shown in prior art FIG. 1 repetitively transmits certain information both in the preamble portion and at regular intervals within the encrypted voice data stream to permit a receiver to initially synchronize with a transmitter despite the usual Raleigh fading which may be expected on radio frequencies communications channels and also to permit "late entry" (in the event that the preamble is "missed" or unsuccessively decoded) and/or recovery of synchronization (in the event that synchronization initially acquired from the preamble is subsequently lost before the end of a message).
In the signalling protocol shown in FIG. 1, initial frame synchronization, ongoing frame synchronization, repeater addressing, cryptographic synchronization and selective signalling signals are all repetitively transmitted in the relatively long preamble portion for fade protection, and are also repetitively retransmitted at regular intervals within the subsequent encrypted voice data stream. Due to the placement of and repetitions of the various control fields within the FIG. 1 protocol, the protocol provides a very high probability of correct initial synchronization and addressing functions. PG,5
The preamble portion of the FIG. 1 signalling protocol preferably includes: (a) a dotting pattern; (b) a synchronization sequence including a repeated group of synchronization signals; and (c) an initialization vector (IV) and selective signalling (SS) sequence (which includes repeated selective signalling, initialization vector and guardband (GB) data signals).
Each data frame of the FIG. 1 protocol includes a "header" portion, a message portion, and an end of message portion. The header portion includes versions of the synchronization sequence and the IV and SS sequence transmitted within the preamble portion. The message portion includes digital signals to be communicated (e.g., encrypted voice data). The transmitted message terminates in an end of message (EOM) word including a synchronization field and a dotting pattern.
The dotting sequence in the FIG. 1 preamble portion is preferably an alternating 1,0 pattern of digital signals (e.g., 10101010 . . . ) continued for 240 bits (25 milliseconds at 9600 baud). This dotting pattern allows circuits within the receiver of the communications system to quickly obtain bit synchronization.
The synchronization sequence occurring within the preamble portion after the dotting pattern includes three repeated fields: a 16 bit synchronization word "S" (preferably an 11 bit Barker code such as 11100010010 and 5 bits of "fill" or dotting), an 8 bit "outside address" (OA) repeated once in complimented form to complete a second 16 bit field; and a 5 bit synchronization number (SN) repeated three times (with the second repeat being in complimented form) plus one final bit of odd parity code so as to complete the third 16 bit field.
The repeated IV and SS sequence following the synchronization sequence includes a 64 bit guardband (GB), a 64 bit initialization vector (IV), and a 16 bit selective signalling address (SS). In the prior art FIG. 1 protocol, the 64 bit guardband GB provides fade protection (and is not used to carry useful intelligence), and the 64 bit IV field establishes cryptographic synchronization in accordance with the conventional DES (described, for example, in "Federal Information Processing Standards" publication no. 46, Data Encryption Standard, U.S. Department of Commerce, NTIS, 5285 Port Royal Road, Springfield, Va. 22161). The 16 bit selective signalling field SS provides group and individual selective signalling capability within a radio communication network (i.e., "addresses" specifying particular individual or groups of receivers are transmitted in this field). The IV, GB and SS fields are repeated nine times in the FIG. 1 protocol.
Following the preamble are successive data frames each of which preferably includes a subpreamble ("header") portion and successive bits of digital data signals (e.g., encrypted voice data). The header includes a single repeat of the synchronization word S, the outside address field OA, the initialization vector IV and the selective signalling address SS. Enough information is provided in each header portion so as to allow for late entry into an ongoing message or conversation and/or so as to reestablish lost frame or cryptographic synchronization (e.g., as might occur from temporary loss of signal due to fading or multipath interference conditions or the like on a typical radio frequency communications channel). A synchronization maintenance control function in the receiver monitors the ongoing received data frame header--and can reestablish bit synchronization, frame synchronization, cryptographic synchronization and selective signalling control from the header portion alone.
An end of message (EOM) signal is provided at the end of a message transmission to alert receivers that the message is terminated.
The FIG. 1 signalling protocol is highly successful, and permits extremely reliable communication of digital signals over a radio (or other) communications channel subject to fading, noise and other phenomenon at a sufficient data rate and with very low error probability. However, further improvements are possible.
For example, although the FIG. 1 signalling protocol is designed to communicate encrypted digitized voice data (although it is by no means limited to communicating this type of information), it would be desirable to selectively communicate digitized voice data or digital information provided by a purely digital signal source such as a data terminal--and to provide signalling control signals within the communications protocol to signify to the receiver what type of message information is being communicated. There exists a great demand for radio transceivers which can convey not only voice information but also digital information produced by a data terminal or computer. While the FIG. 1 protocol is not limited to communicating voice data (virtually any type of digital data could be conveyed within the message data frames), a transmitted indicator signal indicating the type of data being transmitted would permit receivers to treat received digital information in the appropriate manner (e.g., convert the data to analog audio signals for application to a loudspeaker, or preserve the data in digital form to be displayed on a data terminal or stored in a computer memory).
Further improvement in error-free transmission at high data rates is also possible. An effective data rate of 9600 baud on an error free channel is desired. This effective data rate should degrade to no less than 2400 baud on a 1.0% BER channel with no RF fading. The probability of receiving a message of 9600 bits incorrectly on a channel with 1.0% BER and no fading should be no greater than 0.0001, and with typical fading on the channel, this probability should increase to no more than 0.01.
The signalling protocol should also demonstrate some form of adaptivity to deleterious phenomenon present on the communications channel (e.g., noise and/or fading). While numerous repetitions of the same data may be necessary to ensure accurate reception when the communications channel is subject to noise and/or fading, such repetitions decrease the effective data rate and may not be necessary when communicating signals over an optimal channel having little or no fading and noise. If the receiver has trouble receiving a particular data packet, then the transmitter should somehow adapt and repeat the packet. Data packets should be repeated more often as the bit error rate of the communications channel increases--without significantly increasing the "overhead" traffic on the channel used for communicating control signals rather than useful data signals.
Moreover, it would be desirable for such an adaptive signal format to be compatible with the prior signalling format shown in FIG. 1 (and thus, with existing communications equipment such as repeaters and mobile transceivers designed to communicate using that protocol).